(Reuters) - Former National Security Agency contractor Edward Snowden has offered to collaborate with a Brazilian investigation into the NSA surveillance program he revealed earlier this year, according to a letter published in a local newspaper on Tuesday.

In "An Open Letter to the Brazilian People," published by newspaper Folha De S. Paulo, Snowden said he would like to assist in a congressional probe into the NSA's spying program, which monitored the personal communications of President Dilma Rousseff and other Brazilians.

"I have expressed my willingness to assist wherever appropriate and lawful, but unfortunately the United States government has worked very hard to limit my ability to do so," the letter said.

Snowden is living in Russia under temporary asylum that is due to expire in August. He had previously requested asylum in Brazil, but that request remains unanswered. While he stopped short of asking again in the letter, he suggested that any collaboration with Brazilian authorities would depend on it.

"Until a country grants permanent political asylum, the U.S. government will continue to interfere with my ability to speak," Snowden said.

The revelations of NSA spying damaged relations between the United States and Latin America's largest economy and prompted Rousseff to cancel a state visit to Washington in October and become a global advocate for curbs on internet surveillance.

The Brazilian Senate is investigating the extent of NSA spying on the country, and congressmen there have sought to interview Snowden.

The original version of the letter in English was also published on the Facebook page of David Miranda, partner of journalist and blogger Glenn Greenwald, who first brought the Snowden leaks to the world's attention.

Miranda started a campaign on the Avaaz petition site calling for Brazil to offer Snowden asylum. Avaaz said more than 2,600 people had already backed the petition.

Press representatives from Brazil's foreign ministry and presidency were not available for comment.

In the letter, Snowden also praised Brazil's efforts in the United Nations to limit excessive electronic surveillance.

Last month a U.N. General Assembly committee expressed concern at the harm such scrutiny, including spying in foreign states and the mass collection of personal data, might have on human rights, following a joint resolution introduced by Brazil and Germany.

On Monday, White House spokesman Jay Carney dismissed the suggestion that the United States could grant amnesty to Snowden if he were to turn over the documents in his possession.